I. Field
The present invention relates generally to communication, and more specifically to techniques for scanning frequency channels to search for candidate base stations to potentially handoff in a wireless communication system.
II. Background
A wireless device (e.g., a cellular phone) in a wireless communication system is typically designed to operate in one of several modes, such as “active” and “idle”, at any given moment. In the active mode, the wireless device can actively exchange data with one or more base stations in the system, e.g., for a voice or data call. In the idle mode, the wireless device typically monitors a paging channel for messages applicable to the device. Such messages may include page messages that alert the wireless device to the presence of an incoming call and overhead messages that carry system and other information for the device.
In the idle mode, a wireless device continues to consume power to sustain circuitry needed to monitor the signals transmitted by the base stations in the system. The wireless device may be portable and powered by an internal battery. Power consumption by the device in the idle mode decreases the available battery power, which then shortens the standby time between battery recharges and the talk time when a call is placed or received. Therefore, it is highly desirable to minimize power consumption in the idle mode to increase battery life.
In one common technique for reducing power consumption in the idle mode, user-specific messages are sent on the paging channel to the wireless device at designated times, if at all. The paging channel is divided into numbered “slots”. The wireless device is then assigned specific slots on which it may receive user-specific messages. With such a slotted paging channel, the wireless device can operate in a “slotted” mode whereby it periodically, rather than continuously, monitors the paging channel for messages from base stations in the system. In the slotted mode, the wireless device wakes up from a “sleep” state prior to its assigned slot, enters an “awake” state and processes the paging channel for messages, and reverts back to the sleep state if additional communication is not required. The wireless device remains in the awake state if a received message requires the device to perform additional tasks. In the time period between successive presences in the awake state, the wireless device is in the sleep state and the base stations do not send any pages or user-specific messages to the wireless device. The wireless device powers down as much circuitry as possible in the sleep state to conserve battery power.
The base stations in the system may transmit data and messages on multiple frequency channels. To enhance performance, it is desirable for the wireless device to scan these multiple frequency channels for candidate base stations while in the slotted mode. The wireless device may choose to listen to a specific base station (usually the base station with the highest signal-to-noise ratio (SNR) at the device) and designate this base station as the “active” base station. A “candidate” base station is one that is deemed to be “better” than the active base station and to which the wireless device may handoff to receive communication service, if all handoff criteria are satisfied. Another base station may be deemed to be better than the active base station if (1) the signal from this other base station is measured with a higher SNR at the wireless device than that of the active base station and (2) the higher SNR is above a predetermined threshold. The wireless device can typically process only one frequency channel at a time and is normally tuned to one of the frequency channels supported by the active base station. The wireless device would need to tune its RF circuitry to each different frequency channel in order to scan for candidate base stations on that frequency channel. The tuning and scanning for candidate base stations on other frequency channels may significantly deplete battery power and reduce standby time.
There is therefore a need in the art for techniques to scan other frequency channels for candidate base stations while minimizing battery consumption.